In recent years, zoom lens (i.e., variable magnification) optical systems (hereinafter, simply "zoom lenses") have come into common use as photographic lenses in lens-shutter-type cameras, and in particular, zoom ratios in excess of two are rapidly becoming standard.
Zoom lenses with high zoom ratios frequently employ three or more movable lens groups, the main reason is that such zoom lenses offer a large capability for correcting aberrations as compared with two-group zoom lenses so that the lenses increase the selectivity of the loci of movement of the respective lens groups while zooming from the extreme wide-angle state to the extreme telephoto state. This large number stems from the increase in the number of degrees of freedom of the loci of movement of the respective lens groups in changing (i.e., "zooming") from the extreme wide-angle state to the extreme telephoto state as compared with two-group zoom lenses. Another reason is that variations in off-axis aberrations accompanying changes in the focal length (i.e., positional) state of the lens can be satisfactorily corrected because there is a relatively small change in lateral magnification with a change in the positional states of the lens groups.
The positive-positive-negative three-group type of zoom lens is conventionally known to possess a comparatively small number of lenses and to be suited to applications requiring compactness. Focusing for such zoom lenses may be accomplished by movement of either all or a part of the lens system. A zoom lens where focusing is accomplished as a result of moving only part of the lens system is disclosed in Japanese Laid-Open Patent Application (Kokai) No. Hi [1989]-204013 and in Japanese Laid-Open Patent Application (Kokai) No. H2 [1990]-16515.
In zoom lenses where the entire lens system is moved during focusing, one focusing method involves moving the entire lens system in unison, and another focusing method involves moving the lens groups in different ratios. In the latter focusing method, it is known to carry out focusing such that the ratio or ratios governing the amounts by which the respective lens groups move remain(s) constant over the entire range of positional states from the extreme wide-angle state to the extreme telephoto state.
In the focusing methods disclosed at the aforementioned Japanese Laid-Open Patent Applications (Kokai) No. H1 [1989]-204013 and No. H2[1990]-16515 focusing is carried out using lens groups to the rear (i.e., imagewise) of the first lens group. Unfortunately, with this system, it has proven impossible to keep variations in aberrations to a minimum during focusing, and to maintain good imaging performance over all the positional states from the extreme wide-angle state to the extreme telephoto state. The main reason for this is the large variation in the location at which off-axis light beams pass through the first (i.e., most objectwise) lens group during focusing.
In the focusing method where the entire lens system moves in unison, it has proven impossible to keep variations in aberration to a minimum, and to maintain good imaging performance. This is due to the large variation in the location at which off-axis light beams pass through the lens groups to the front and rear of and on either side of the aperture stop during focusing.
In the focusing method where the lens groups move in different ratios, it is known to achieve focusing where the ratio(s) governing the amounts by which the respective lens groups move constant over the entire range of positional states from the extreme wide-angle state to the extreme telephoto state. However, it has been impossible to keep variations in aberration to a minimum during focusing, and to maintain good imaging performance, over the entire range of magnifications. The main reason for this is that the locations at which off-axis light beams pass through the lens groups to the front and rear of and on either side of the aperture stop vary greatly with the change in magnification of the zoom lens when going from the extreme wide-angle state to the extreme telephoto state.